A modern torpedo is a self-propelled weapon with an explosive warhead, launched above or below the water surface, propelled underwater towards a target, and designed to detonate either on contact with its target or in proximity to it.

Historically, it was called an automotive, automobile, locomotive or fish torpedo; colloquially called a fish. The term torpedo was originally employed for a variety of devices, most of which would today be called mines. From about 1900, torpedo has been used strictly to designate an underwater self-propelled weapon.

While the battleship had evolved primarily around engagements between armoured ships with large-caliber guns, the torpedo allowed torpedo boats and other lighter surface ships, submersibles, even ordinary fishing boats or frogmen, and later, aircraft, to destroy large armoured ships without the need of large guns, though sometimes at the risk of being hit by longer-range shellfire.

Today's torpedoes can be divided into lightweight and heavyweight classes; and into straight-running, autonomous homers, and wire-guided. They can be launched from a variety of platforms.

In modern language, a 'torpedo' is an underwater self-propelled explosive, but historically, the term also applied to primitive naval mines. These were used on an ad hoc basis during the early modern period up to the late 19th century. Early spar torpedoes were created by the Dutchman Cornelius Drebbel in the employ of King James I of England; he attached explosives to the end of a beam affixed to one of his own submarines and they were used (to little effect) during the English expeditions to La Rochelle in 1626.[3]

In the early 1800s, the American inventor Robert Fulton, while in France, "conceived the idea of destroying ships by introducing floating mines under their bottoms in submarine boats." He coined the term "torpedo" in reference to the explosive charges with which he outfitted his submarine Nautilus. However, both the French and the Dutch governments were uninterested in the submarine. Fulton then concentrated on developing the torpedo independent of a submarine deployment. On 15 October 1805, while in England, Fulton put on a public display of his "infernal machine", sinking the brig Dorothea with a submerged bomb filled with 180 lb (82 kg) of gunpowder and a clock set to explode in 18 minutes. However, the British government refused to purchase the invention, stating they did not wish to "introduce into naval warfare a system that would give great advantage to weaker maritime nations." Fulton carried out a similar demonstration for the US government on 20 July 1807, destroying a vessel in New York's harbor. Further development languished as Fulton focused on his "steam-boat matters". During the War of 1812, torpedoes were employed in attempts to destroy British vessels and protect American harbors. In fact a submarine-deployed torpedo was used in an unsuccessful attempt to destroy HMS Ramillies while in New London's harbor. This prompted the British Capt. Hardy to warn the Americans to cease efforts with the use of any "torpedo boat" in this "cruel and unheard-of warfare", or he would "order every house near the shore to be destroyed."[2]

During the American Civil War, the term torpedo was used for what is today called a contact mine, floating on or below the water surface using an air-filled demijohn or similar flotation device. These devices were very primitive and apt to prematurely explode. They would be detonated on contact with the ship or after a set time, although electrical detonators were also occasionally used. USS Cairo was the first warship to be sunk in 1862 by an electrically detonated mine. Spar torpedoes were also used; an explosive device was mounted at the end of a spar up to 30 feet (9.1 m) long projecting forward underwater from the bow of the attacking vessel, which would then ram the opponent with the explosives. These were used by the Confederate submarine H. L. Hunley to sink USS Housatonic although the weapon was apt to cause as much harm to its user as to its target. Rear AdmiralDavid Farragut's famous/apocryphal command during the Battle of Mobile Bay in 1864, "Damn the torpedoes, full speed ahead!" refers to a minefield laid at Mobile, Alabama.

In 1866 British engineer Robert Whitehead invented the first effective self-propelled torpedo, the eponymous Whitehead torpedo. French and German inventions followed closely, and the term torpedo came to describe self-propelled projectiles that traveled under or on water. By 1900, the term no longer included mines and booby-traps as the navies of the world added submarines, torpedo boats and torpedo boat destroyers to their fleets.[6][7]

Whitehead was unable to improve the machine substantially, since the clockwork motor, attached ropes, and surface attack mode all contributed to a slow and cumbersome weapon. However, he kept considering the problem after the contract had finished, and eventually developed a tubular device, designed to run underwater on its own, and powered by compressed air. The result was a submarine weapon, the Minenschiff (mine ship), the first modern self-propelled torpedo, officially presented to the Austrian Imperial Naval commission on December 21, 1866.

The first trials were not successful as the weapon was unable to maintain a course at a steady depth. After much work, Whitehead introduced his "secret" in 1868 which overcame this. It was a mechanism consisting of a hydrostatic valve and pendulum that caused the torpedo's hydroplanes to be adjusted so as to maintain a preset depth.

Robert Whitehead (right) invented the modern self-propelled torpedo in 1866. Pictured examining a battered test torpedo in Fiume c. 1875.

After the Austrian government decided to invest in the invention, Whitehead started the first torpedo factory in Fiume. In 1870, he improved the devices to travel up to approximately 1,000 yd (910 m) at a speed of up to 6 kn (11 km/h), and by 1881 the factory was exporting torpedoes to ten other countries. The torpedo was powered by compressed air and had an explosive charge of gun-cotton.[8] Whitehead went on to develop more efficient devices, demonstrating torpedoes capable of 18 kn (33 km/h) in 1876, 24 kn (44 km/h) in 1886, and, finally, 30 kn (56 km/h) in 1890.

Royal Navy representatives visited Fiume for a demonstration in late 1869, and in 1870 a batch of torpedoes was ordered. In 1871, the British Admiralty paid Whitehead £15,000 for certain of his developments and production started at the Royal Laboratories in Woolwich the following year. In 1893, RN torpedo production was transferred to the Royal Gun Factory. The British later established a Torpedo Experimental Establishment at HMS Vernon and a production facility at the Royal Naval Torpedo Factory, Greenock in 1910. These are now closed.

Whitehead opened a new factory near Portland Harbour, England in 1890, which continued making torpedoes until the end of the Second World War. Because orders from the RN were not as large as expected, torpedoes were mostly exported. A series of devices was produced at Fiume, with diameters from 14 in (36 cm) upward. The largest Whitehead torpedo was 18 in (46 cm) in diameter and 19 ft (5.8 m) long, made of polished steel or phosphor bronze, with a 200-pound (91 kg) gun-cotton warhead. It was propelled by a three-cylinder Brotherhood engine, using compressed air at around 1,300 psi (9.0 MPa) and driving two contra-rotating propellers, and was designed to self-regulate its course and depth as far as possible. By 1881, nearly 1500 torpedoes had been produced. Whitehead also opened a factory at St Tropez in 1890 that exported torpedoes to Brazil, Holland, Turkey and Greece.

Whitehead purchased rights to the gyroscope of Ludwig Obry in 1888 but it was not sufficiently accurate, so in 1890 he purchased a better design to improve control of his designs, which came to be called the "Devil's Device". The firm of L. Schwartzkopff in Germany also produced torpedoes and exported them to Russia, Japan and Spain. In 1885, Britain ordered a batch of 50 as torpedo production at home and at Fiume could not meet demand.

By World War I, Whitehead's torpedo remained a worldwide success, and his company was able to maintain a monopoly on torpedo production. By that point, his torpedo had grown to a diameter of 18 inches with a maximum speed of 30.5 knots (56.5 km/h; 35.1 mph) with a warhead weighing 170 pounds (77 kg).

Whitehead faced competition from the American Lieutenant CommanderJohn A. Howell, whose own design, driven by a flywheel, was simpler and cheaper. It was produced from 1885 to 1895, and it ran straight, leaving no wake. A Torpedo Test Station was set up on Rhode Island in 1870. The Howell torpedo was the only United States Navy model until Whitehead torpedoes produced by Bliss and Williams entered service in 1894. Five varieties were produced, all 18 in (46 cm) diameter. The United States Navy started using the Whitehead torpedo in 1892 after an American company, E.W. Bliss, secured manufacturing rights.[9][inconsistent]

HMS Lightning, built in 1877 as a small attack boat armed with torpedoes.

Ships of the line were superseded by ironclads, large steam powered ships with heavy gun armament and heavy armour, in the mid 19th century. Ultimately this line of development led to the dreadnought category of all-big-gun battleship, starting with HMS Dreadnought.

Although these ships were incredibly powerful, the new weight of armour slowed them down, and the huge guns needed to penetrate that armour fired at very slow rates. This allowed for the possibility of a small and fast ship that could attack the battleships, at a much lower cost. The introduction of the torpedo provided a weapon that could cripple, or sink, any battleship.

It was designed to run at a consistent depth of 12 feet (3.7 m), and was fitted with an indicator mast that just broke the surface of the water. At night the mast had a small light, only visible from the rear. Two steel drums were mounted one behind the other inside the torpedo, each carrying several thousands yards of high-tensile steel wire. The drums connected via a differential gear to twin contra-rotating propellers. If one drum was rotated faster than the other, then the rudder was activated. The other ends of the wires were connected to steam-powered winding engines, which were arranged so that speeds could be varied within fine limits, giving sensitive steering control for the torpedo.[11]

The torpedo attained a speed of 20 knots (37 km/h; 23 mph) using a wire .04 inches (1.0 mm) in diameter but later this was changed to .07 inches (1.8 mm) to increase the speed to 27 knots (50 km/h; 31 mph). The torpedo was fitted with elevators controlled by a depth-keeping mechanism, and the fore and aft rudders operated by the differential between the drums.[12]

Brennan travelled to Britain, where the Admiralty examined the torpedo and found it unsuitable for shipboard use. However, the War Office proved more amenable, and in early August 1881 a special Royal Engineer committee was instructed to inspect the torpedo at Chatham and report back directly to the Secretary of State for War, Hugh Childers. The report strongly recommended that an improved model be built at government expense. In 1883 an agreement was reached between the Brennan Torpedo Company and the government. The newly appointed Inspector-General of Fortifications in England, Sir Andrew Clarke, appreciated the value of the torpedo and in spring 1883 an experimental station was established at Garrison Point Fort, Sheerness on the River Medway and a workshop for Brennan was set up at the Chatham Barracks, the home of the Royal Engineers. Between 1883 and 1885 the Royal Engineers held trials and in 1886 the torpedo was recommended for adoption as a harbour defence torpedo. It was used throughout the British Empire for more than fifteen years.[12]

Around 1897, Nikola Tesla patented a remote controlled boat and later demonstrated the feasibility of radio-guided torpedoes to the United States military, only to be turned down.

Several western sources reported that the Qing dynasty Imperial Chinese military, under the direction of Li Hongzhang, acquired electric torpedoes, which they deployed in numerous waterways, along with fortresses and numerous other modern military weapons acquired by China.[14] At the Tientsin Arsenal in 1876, the Chinese developed the capacity to manufacture these "electric torpedoes" on their own.[15] Although a form of Chinese art, the Nianhua, depict such torpedoes being used against Russian ships during the Boxer Rebellion, whether they were actually used in battle against them is undocumented and unknown.[16]

The Russo-Japanese War (1904–1905) was the first great war of the 20th century.[17] During the course of the war the Imperial Russian and Imperial Japanese navies launched nearly 300 torpedoes at each other, all of them of the "self propelled automotive" type.[18] The deployment of these new underwater weapons resulted in only one battleship, but two armored cruisers and two destroyers being sunk in action; with the remainder of the 80 some odd warships being sunk by the more conventional methods of gunfire, mines, and scuttling.[19]

On 27 May 1905, during the battle of Tsushima, Admiral Rozhestvensky's flagship, the battleship Knyaz Suvorov, had been gunned to a wreck by Admiral Togo's 12 inch gunned battleline. With the Russians sunk and scattering, Togo prepared for pursuit, and while doing so ordered his torpedo boat destroyers (TBDs) (mostly referred to as just destroyers in most written accounts) to finish off the Russian battleship. The Knyaz Suvorov was set upon by 17 torpedo firing warships, ten of which were destroyers and four torpedo boats. 21 torpedoes were launched at the pre-dreadnought, and three struck home, one fired from the destroyer IJN Murasame and two from torpedo boats #72 and #75.[20] The flagship slipped under the waves shortly thereafter, taking over 900 men with her to the bottom.[21]

The end of the war[which?] fuelled new theories, and the idea of dropping lightweight torpedoes from aircraft was conceived in the early 1910s by Bradley A. Fiske, an officer in the United States Navy.[22] Awarded a patent in 1912,[23][24] Fiske worked out the mechanics of carrying and releasing the aerial torpedo from a bomber, and defined tactics that included a night-time approach so that the target ship would be less able to defend itself. Fiske determined that the notional torpedo bomber should descend rapidly in a sharp spiral to evade enemy guns, then when about 10 to 20 feet (3 to 6 m) above the water the aircraft would straighten its flight long enough to line up with the torpedo's intended path. The aircraft would release the torpedo at a distance of 1,500 to 2,000 yards (1,400 to 1,800 m) from the target.[22] Fiske reported in 1915 that, using this method, enemy fleets could be attacked within their own harbors if there were enough room for the torpedo track.[25]

Meanwhile, the Royal Naval Air Service began actively experimenting with this possibility. The first successful aerial torpedo drop was performed by Gordon Bell in 1914 – dropping a Whitehead torpedo from a Short S.64seaplane. The success of these experiments led to the construction of the first purpose-built operational torpedo aircraft, the Short Type 184, built from 1915.[26]

On 17 August 1915 Flight Commander Edmonds torpedoed and sank a Turkish transport ship a few miles north of the Dardanelles. His formation colleague, Flt Lt G B Dacre, was forced to land on the water owing to engine trouble but, seeing an enemy tug close by, taxied up to it and released his torpedo, sinking the tug. Without the weight of the torpedo Dacre was able to take off and return to the Ben-My-Chree.[29]

Torpedoes were widely used in the First World War, both against shipping and against submarines[30]. Germany disrupted the supply lines to Britain largely by use of submarine torpedoes, though submarines also extensively used guns. Britain and its allies also used torpedoes throughout the war. U-boats themselves were often targeted, twenty being sunk by torpedo[31]. Two Royal Italian Navy torpedo boats scored a success against an Austrian-Hungarian squadron, sinking the battleship Szent István with two torpedoes.

Initially the Imperial Japanese Navy purchased Whitehead or Schwartzkopf torpedoes but by 1917 they were conducting experiments with pure oxygen instead of compressed air. Because of explosions they abandoned the experiments but resumed them in 1926 and by 1933 had a working torpedo. They also used conventional wet-heater torpedoes.

In the inter-war years, tight budgets caused nearly all navies to skimp on testing their torpedoes. As a result, only the Japanese had fully tested torpedoes (in particular the Type 93, nicknamed Long Lance postwar by historian Samuel E. Morison)[32][33] at the start of World War II. The lack of reliability caused major problems for the American submarine force in the early years of the American involvement in World War II, primarily in the Pacific Theater. One possible exception to the "lack of development" scenario was the 45-cm calibre, 1931-premiered Japanese Type 91 torpedo, the sole aerial torpedo (Koku Gyorai) developed and brought into service by the Japanese Empire before World War II;[34] its advanced PID controller and use of jettisonable, wood-structure Kyoban aerial stabilizing surfaces which released upon water-entry from its deploying torpedo bomber, made it a formidable enough Axis Powers-deployed anti-ship aerial weapon for Nazi Germany to consider manufacture of as the Luftorpedo LT 850[35]after August 1942.

Many classes of surface ships, submarines, and aircraft were armed with torpedoes. Naval strategy at the time was to use torpedoes, launched from submarines or warships, against enemy warships in a fleet action on the high seas. There was concern torpedoes would be ineffective against warships' heavy armor; an answer to this was to detonate torpedoes underneath a ship, badly damaging its keel and the other structural members in the hull, commonly called "breaking its back". This was demonstrated by magnetic influence mines in World War I. The torpedo would be set to run at a depth just beneath the ship, relying on a magnetic exploder to activate at the appropriate time.

Germany, Britain and the U.S. independently devised ways to do this; German and American torpedoes, however, suffered problems with their depth-keeping mechanisms, coupled with faults in magnetic pistols shared by all designs. Inadequate testing had failed to reveal the effect of the Earth's magnetic field on ships and exploder mechanisms, which resulted in premature detonation. The Kriegsmarine and Royal Navy promptly identified and eliminated the problems. In the United States Navy, there was an extended wrangle over the problems plaguing the Mark 14 torpedo (and its Mark 6 exploder). Cursory trials had allowed bad designs to enter service. Both the Navy Bureau of Ordnance and the United States Congress were too busy protecting their own interests to correct the errors, and fully functioning torpedoes only became available to the USN twenty-one months into the Pacific War.[36]

Torpedo boats, such as MTBs, PT boats, or S-boats, enabled relatively small but fast craft to carry enough firepower, in theory, to destroy a larger ship, though this rarely occurred in practice. The largest warship sunk by torpedoes from small craft in World War II was the British cruiser Manchester, sunk by Italian MAS boats on the night of 12/13 August 1942 during Operation Pedestal. Destroyers of all navies were also armed with torpedoes to attack larger ships. In the Battle off Samar, destroyer torpedoes from the escorts of American task force "Taffy 3" showed effectiveness at defeating armor. Damage and confusion caused by torpedo attacks were instrumental[original research?] in beating back a superior Japanese force of battleships and cruisers.[37] In the Battle of the North Cape in December 1943, torpedo hits from British destroyers Savage and Saumarez slowed German battlecruiserScharnhorst enough for British battleshipDuke of York to catch and sink her, and in May 1945 the British 26th Destroyer Flotilla (coincidentally led by Saumarez again) ambushed and sank Japanese heavy cruiser Haguro.

Because of improved submarine strength and speed, torpedoes had to be given improved warheads and better motors. During the Cold War torpedoes were an important asset with the advent of nuclear-powered submarines, which did not have to surface often, particularly those carrying strategic nuclear missiles.

A number of navies have launched torpedo strikes since World War II, including:

During the Korean War the United States Navy successfully attacked a dam with air-launched torpedoes.[38]

On 26 March 2010 the South Korean Navy ship ROKS Cheonan was sunk with the loss of 46 personnel. Subsequent investigation concluded that the warship had been sunk by a North Korean torpedo fired by a midget submarine.

The Whitehead torpedo of 1866, the first successful self-propelled torpedo, used compressed air as its energy source. The air was stored at pressures of up to 2.55 MPa (370 psi) and fed to a piston engine that turned a single propeller at about 100 rpm. It could travel about 180 metres (200 yd) at an average speed of 6.5 knots (12.0 km/h). The speed and range of later models was improved by increasing the pressure of the stored air. In 1906 Whitehead built torpedoes that could cover nearly 1,000 metres (1,100 yd) at an average speed of 35 knots (65 km/h).

At higher pressures the adiabatic cooling experienced by the air as it expanded in the engine caused icing problems. This drawback was remedied by heating the air with seawater before it was fed to the engine, which increased engine performance further, because the air expanded even more after heating. This was the principle used by the Brotherhood engine.

Passing the air through an engine led to the idea of injecting a liquid fuel, like kerosene, into the air and igniting it. In this manner the air is heated more and expands even further, and the burned propellant adds more gas to drive the engine. Construction of such heated torpedoes started circa 1904 by Whitehead's company.

A further improvement was the use of water to cool the combustion chamber of the fuel-burning torpedo. This not only solved heating problems so more fuel could be burned but also allowed additional power to be generated by feeding the resulting steam into the engine together with the combustion products. Torpedoes with such a propulsion system became known as wet heaters, while heated torpedoes without steam generation were retrospectively called dry heaters. A simpler system was introduced by the British Royal Gun factory in 1908. Most torpedoes used in World War I and World War II were wet-heaters.

The amount of fuel that can be burned by a torpedo engine (i.e. wet engine) is limited by the amount of oxygen it can carry. Since compressed air contains only about 21% oxygen, engineers in Japan developed the Type 93 (nicknamed "Long Lance" postwar)[32] for destroyers and cruisers in the 1930s. It used pure compressed oxygen instead of compressed air and had performance unmatched by any contemporary torpedo in service, through the end of World War II. However, oxygen systems posed a serious danger to any ship that came under attack while still carrying such torpedoes; Japan lost several cruisers partly due to catastrophic secondary explosions of Type 93s. During the war, Germany experimented with hydrogen peroxide for the same purpose.

The Brennan torpedo had two wires wound around internal drums. Shore-based steam winches pulled the wires, which spun the drums and drove the propellers. An operator controlled the relative speeds of the winches, providing guidance. Such systems were used for coastal defence of the British homeland and colonies from 1887 to 1903 and were purchased by, and under the control of, the Army as opposed to the Navy. Speed was about 25 knots (46 km/h) for over 2,400 m.

The Howell torpedo used by the US Navy in the late 19th century featured a heavy flywheel that had to be spun up before launch. It was able to travel about 400 yards (370 m) at 25 knots (46 km/h). The Howell had the advantage of not leaving a trail of bubbles behind it, unlike compressed air torpedoes. This gave the target vessel less chance to detect and evade the torpedo, and avoided giving away the attacker's position. Additionally, it ran at a constant depth, unlike Whitehead models.

Electric propulsion systems avoided tell-tale bubbles. John Ericsson invented an electrically propelled torpedo in 1873; it was powered by a cable from an external power source, as batteries of the time had insufficient capacity. The Sims-Edison torpedo was similarly powered. The Nordfelt torpedo was also electrically powered and was steered by impulses down a trailing wire.

Germany introduced its first battery-powered torpedo shortly before World War II, the G7e. It was slower and had shorter range than the conventional G7a, but was wakeless and much cheaper. Its lead-acidrechargeable battery was sensitive to shock, required frequent maintenance before use, and required preheating for best performance. The experimental G7ep, an enhancement of the G7e, used primary cells.

The United States had an electric design, the Mark 18, largely copied from the German torpedo (although with improved batteries), as well as FIDO, an air-dropped acoustic homing torpedo for anti-submarine use.

Modern electric torpedoes such as the Mark 24 Tigerfish or DM2 series commonly use silver oxide batteries that need no maintenance, so torpedoes can be stored for years without losing performance.

A number of experimental rocket-propelled torpedoes were tried soon after Whitehead's invention but were not successful. Rocket propulsion has been implemented successfully by the Soviet Union, for example in the VA-111 Shkval—and has been recently revived in Russian and German torpedoes, as it is especially suitable for supercavitating devices.[40]

The first of Whitehead's torpedoes had a single propeller and needed a large vane to stop it spinning about its longitudinal axis. Not long afterward the idea of contra-rotating propellers was introduced, to avoid the need for the vane. The three-bladed propeller came in 1893 and the four-bladed one in 1897. To minimise noise, today's torpedoes often use pump-jets.

Torpedoes may be aimed at the target and fired unguided, similarly to an artillery shell, or they may be guided onto the target. They may be guided automatically towards the target by some procedure, e.g., sound (homing), or by the operator, typically via commands sent over a signal-carrying cable (wire guidance).

The Victorian era Brennan torpedo could be steered onto its target by varying the relative speeds of its propulsion cables. However, the Brennan required a substantial infrastructure and was not suitable for shipboard use. Therefore, for the first part of its history, the torpedo was guided only in the sense that its course could be regulated so as to achieve an intended impact depth (because of the sine wave running path of the Whitehead,[41] this was a hit or miss proposition, even when everything worked correctly) and, through gyroscopes, a straight course. With such torpedoes the method of attack in small torpedo boats, torpedo bombers and small submarines was to steer a predictable collision course abeam to the target and release the torpedo at the last minute, then veer away, all the time subject to defensive fire.

In larger ships and submarines, fire control calculators gave a wider engagement envelope. Originally, plotting tables (in large ships), combined with specialised slide rules (known in U.S. service as the "banjo" and "Is/Was"),[42] reconciled the speed, distance, and course of a target with the firing ship's speed and course, together with the performance of its torpedoes, to provide a firing solution. By the Second World War, all sides had developed automatic electro-mechanical calculators, exemplified by the U.S. Navy's Torpedo Data Computer.[43] Submarine commanders were still expected to be able to calculate a firing solution by hand as a backup against mechanical failure, and because many submarines existing at the start of the war were not equipped with a TDC; most could keep the "picture" in their heads and do much of the calculations (simple trigonometry) mentally, from extensive training.[44]

Against high-value targets and multiple targets, submarines would launch a spread of torpedoes, to increase the probability of success. Similarly, squadrons of torpedo boats and torpedo bombers would attack together, creating a "fan" of torpedoes across the target's course. Faced with such an attack, the prudent thing for a target to do was to turn so as to parallel the course of the incoming torpedo and steam away from the torpedoes and the firer, allowing the relatively short range torpedoes to use up their fuel. An alternative was to "comb the tracks", turning to parallel the incoming torpedo's course, but turning towards the torpedoes. The intention of such a tactic was still to minimise the size of target offered to the torpedoes, but at the same time be able to aggressively engage the firer. This was the tactic advocated by critics of Jellicoe's actions at Jutland, his caution at turning away from the torpedoes being seen as the reason the Germans escaped.

The use of multiple torpedoes to engage single targets depletes torpedo supplies and greatly reduces a submarine's combat endurance.[45] Endurance can be improved by ensuring a target can be effectively engaged by a single torpedo, which gave rise to the guided torpedo.

In WW2 the Germans introduced programmable pattern running torpedoes, which would run a predetermined pattern until they either ran out of fuel, or hit something. The earlier version, FaT, ran out after launch in a straight line, and then weaved backwards and forwards parallel to that initial course, whilst the more advanced LuT could transit to a different angle after launch, and then enter a more complex weaving pattern.[46]

Though Luppis' original design had been rope guided, torpedoes were not wire-guided until the 1960s.

During the First World War the U.S. Navy evaluated a radio controlled torpedo launched from a surface ship called the Hammond Torpedo.[47] A later version tested in the 1930s was claimed to have an effective range of 6 miles.[48]

Modern torpedoes use an umbilical wire, which nowadays allows the computer processing-power of the submarine or ship to be used. Torpedoes such as the U.S. Mark 48 can operate in a variety of modes, increasing tactical flexibility.

Homing "fire and forget" torpedoes can use passive or active guidance, or a combination of both. Passive acoustic torpedoes home in on emissions from a target. Active acoustic torpedoes home in on the reflection of a signal, or "ping", from the torpedo or its parent vehicle; this has the disadvantage of giving away the presence of the torpedo. In semi-active mode, a torpedo can be fired to the last known position or calculated position of a target, which is then acoustically illuminated ("pinged") once the torpedo is within attack range.

The homing systems for torpedoes are generally acoustic, though there have been other target sensor types used. A ship's acoustic signature is not the only emission a torpedo can home in on: to engage U.S. supercarriers, the Soviet Union developed the 53–65wake-homing torpedo. As standard acoustic lures can't distract a wake homing torpedo, the US Navy has installed the Surface Ship Torpedo Defense on aircraft carriers that uses a Countermeasure Anti-Torpedo to home in on and destroy the attacking torpedo.[49]

The warhead is generally some form of aluminised explosive, because the sustained explosive pulse produced by the powdered aluminium is particularly destructive against underwater targets. Torpex was popular until the 1950s, but has been superseded by PBX compositions. Nuclear warheads for torpedoes have also been developed, e.g. the Mark 45 torpedo. In lightweight antisubmarine torpedoes designed to penetrate submarine hulls, a shaped charge can be used. Detonation can be triggered by direct contact with the target or by a proximity fuze incorporating sonar and/or magnetic sensors.

When a torpedo with a contact fuze strikes the side of the target hull, the resulting explosion creates a bubble of expanding gas, the walls of which move faster than the speed of sound in water, thus creating a shock wave. The side of the bubble which is against the hull rips away the external plating creating a large breach. The bubble then collapses in on itself, forcing a high-speed stream of water into the breach which can destroy bulkheads and machinery in its path.[50]

A torpedo fitted with a proximity fuze can be detonated directly under the keel of a target ship. The explosion creates a gas bubble which may damage the keel or underside plating of the target. However, the most destructive part of the explosion is the upthrust of the gas bubble, which will bodily lift the hull in the water. The structure of the hull is designed to resist downward rather than upward pressure, causing severe strain in this phase of the explosion. When the gas bubble collapses, the hull will tend to fall into the void in the water, creating a sagging effect. Finally, the weakened hull will be hit by the uprush of water caused by the collapsing gas bubble, causing structural failure. On vessels up to the size of a modern frigate, this can result in the ship breaking in two and sinking. This effect is likely to prove less catastrophic on a much larger hull, for instance that of an aircraft carrier.[50]

The damage that may be caused by a torpedo depends on the "shock factor value", a combination of the initial strength of the explosion and of the distance between the target and the detonation. When taken in reference to ship hull plating, the term "Hull Shock Factor" (HSF) is used, while keel damage is termed "Keel Shock Factor" (KSF). If the explosion is directly underneath the keel, then HSF is equal to KSF, but explosions that are not directly underneath the ship will have a lower value of KSF.[51]

Usually only created by contact detonation, direct damage is a hole blown in the ship. Among the crew, fragmentation wounds are the most common form of damage. Flooding typically occurs in one or two main watertight compartments, which can sink smaller ships or disable larger ones.

The bubble jet effect occurs when a mine or torpedo detonates in the water a short distance away from the targeted ship. The explosion creates a bubble in the water, and due to the difference in pressure, the bubble will collapse from the bottom. The bubble is buoyant, and so it rises towards the surface. If the bubble reaches the surface as it collapses, it can create a pillar of water that can go over a hundred meters into the air (a "columnar plume"). If conditions are right and the bubble collapses onto the ship's hull, the damage to the ship can be extremely serious; the collapsing bubble forms a high-energy jet that can break a metre-wide hole straight through the ship, flooding one or more compartments, and is capable of breaking smaller ships apart. The crew in the areas hit by the pillar are usually killed instantly. Other damage is usually limited.[51]

If the torpedo detonates at a distance from the ship, and especially under the keel, the change in water pressure causes the ship to resonate. This is frequently the most deadly type of explosion, if it is strong enough. The whole ship is dangerously shaken and everything on board is tossed around. Engines rip from their beds, cables from their holders, etc. A badly shaken ship usually sinks quickly, with hundreds, or even thousands of small leaks all over the ship and no way to power the pumps. The crew fare no better, as the violent shaking tosses them around.[51] This shaking is powerful enough to cause disabling injury to knees and other joints in the body, particularly if the affected person stands on surfaces connected directly to the hull (such as steel decks).

The resulting gas cavitation and shock-front-differential over the width of the human body is sufficient to stun or kill divers.[54]

Control surfaces are essential for a torpedo to maintain its course and depth. A homing torpedo also needs to be able to outmanoeuvre a target. Good hydrodynamics are needed for it to attain high speed efficiently and also to give long range, since the torpedo has limited stored energy.

Torpedoes may be launched from submarines, surface ships, helicopters and fixed-wing aircraft, unmanned naval mines and naval fortresses.[55] They are also used in conjunction with other weapons; for example the Mark 46 torpedo used by the United States is the warhead section of the ASROC (Anti-Submarine ROCket) and the CAPTOR mine (CAPsulated TORpedo) is a submerged sensor platform which releases a torpedo when a hostile contact is detected.

Originally, Whitehead torpedoes were intended for launch underwater and the firm was upset when they found out the British were launching them above water, as they considered their torpedoes too delicate for this. However, the torpedoes survived. The launch tubes could be fitted in a ship's bow, which weakened it for ramming, or on the broadside; this introduced problems because of water flow twisting the torpedo, so guide rails and sleeves were used to prevent it. The torpedoes were originally ejected from the tubes by compressed air but later slow burning gunpowder was used. Torpedo boats originally used a frame that dropped the torpedo into the sea. Royal Navy Coastal Motor Boats of World War I used a rear-facing trough and a cordite ram to push the torpedoes into the water tail-first; they then had to move rapidly out of the way to avoid being hit by their own torpedo.

Developed in the run-up to the First World War,[citation needed] multiple-tube mounts (initially twin, later triple and in WW2 up to quintuple in some ships) for 21 to 24 in (53 to 61 cm) torpedoes in rotating turntable mounts appeared. Destroyers could be found with two or three of these mounts with between five and twelve tubes in total. The Japanese went one better, covering their tube mounts with splinter protection and adding reloading gear (both unlike any other navy in the world),[56] making them true turrets and increasing the broadside without adding tubes and top hamper (as the quadruple and quintuple mounts did). Considering their Type 93s very effective weapons, the IJN equipped their cruisers with torpedoes. The Germans also equipped their capital ships with torpedoes.

Smaller vessels such as PT boats carried their torpedoes in fixed deck mounted tubes using compressed air. These were either aligned to fire forward or at an offset angle from the centerline.

Later, lightweight mounts for 12.75 in (32.4 cm) homing torpedoes were developed for anti-submarine use consisting of triple launch tubes used on the decks of ships. These were the 1960 Mk 32 torpedo launcher in the USA and part of STWS (Shipborne Torpedo Weapon System) in the UK. Later a below-decks launcher was used by the RN. This basic launch system continues to be used today with improved torpedoes and fire control systems.

Modern submarines use either swim-out systems or a pulse of water to discharge the torpedo from the tube, both of which have the advantage of being significantly quieter than previous systems, helping avoid detection of the firing from passive sonar. Earlier designs used a pulse of compressed air or a hydraulic ram.

Early submarines, when they carried torpedoes, were fitted with a variety of torpedo launching mechanisms in a range of locations; on the deck, in the bow or stern, amidships, with some launch mechanisms permitting the torpedo to be aimed over a wide arc. By World War II, designs favoured multiple bow tubes and fewer or no stern tubes. Modern submarine bows are usually occupied by a large sonar array, necessitating midships tubes angled outward, while stern tubes have largely disappeared. The first French and Russian submarines carried their torpedoes externally in Drzewieckidrop collars. These were cheaper than tubes, but less reliable. Both the United Kingdom and United States experimented with external tubes in World War II. External tubes offered a cheap and easy way of increasing torpedo capacity without radical redesign, something neither had time or resources to do prior to, or early in, the war. British T-class submarines carried up to 13 torpedo tubes, up to 5 of them external. America's use was mainly limited to earlier Porpoise-, Salmon-, and Sargo-class boats. Until the appearance of the Tambors, most American submarines only carried 4 bow and either 2 or 4 stern tubes, something many American submarine officers felt provided inadequate firepower.[citation needed] This problem was compounded by the notorious unreliability of the Mark 14 torpedo.

Late in World War II, the U.S. adopted a 16 in (41 cm) homing torpedo (known as "Cutie") for use against escorts. It was basically a modified Mark 24 Mine with wooden rails to allow firing from a 21 in (53 cm) torpedo tube.[57]

Aerial torpedoes may be carried by fixed-wing aircraft, helicopters or missiles. They are launched from the first two at prescribed speeds and altitudes, dropped from bomb-bays or underwing hardpoints.

Although lightweight torpedoes are fairly easily handled, the transport and handling of heavyweight ones is difficult, especially in the small space of a submarine. After the Second World War, some Type XXI submarines were obtained from Germany by the United States and Britain. One of the main novel developments seen was a mechanical handling system for torpedoes. Such systems were widely adopted as a result of this discovery.[citation needed]

As the final stage of a compound rocket or ramjet powered munition (sometimes called an assisted torpedo).

Many navies have two weights of torpedoes:

A light torpedo used primarily as a close attack weapon, particularly by aircraft.

A heavy torpedo used primarily as a standoff weapon, particularly by submerged submarines.

In the case of deck or tube launched torpedoes, the diameter of the torpedo is obviously a key factor in determining the suitability of a particular torpedo to a tube or launcher, similar to the caliber of the gun. The size is not quite as critical as for a gun, but diameter has become the most common way of classifying torpedoes.

Length, weight, and other factors also contribute to compatibility. In the case of aircraft launched torpedoes, the key factors are weight, provision of suitable attachment points, and launch speed. Assisted torpedoes are the most recent development in torpedo design, and are normally engineered as an integrated package. Versions for aircraft and assisted launching have sometimes been based on deck or tube launched versions, and there has been at least one case of a submarine torpedo tube being designed to fire an aircraft torpedo.

As in all munition design, there is a compromise between standardisation, which simplifies manufacture and logistics, and specialisation, which may make the weapon significantly more effective. Small improvements in either logistics or effectiveness can translate into enormous operational advantages.

^Guinness Book of Air Facts and Feats (3rd ed.). 1977. The first air attack using a torpedo dropped by an aeroplane was carried out by Flight Commander Charles H. K. Edmonds, flying a Short 184 seaplane from Ben-my-Chree on 12 August 1915, against a 5,000 ton Turkish supply ship in the Sea of Marmara. Although the enemy ship was hit and sunk, the captain of a British submarine claimed to have fired a torpedo simultaneously and sunk the ship. It was further stated that the British submarine E14 had attacked and immobilised the ship four days earlier.